BWRs have conventionally utilized active safety systems to control and mitigate accident events. Those events varied from small break to design base accidents. Active systems, consisting of both high-pressure and low-pressure pumping equipment, have been the corner-stones of BWR/4 to BWR/6 safety systems product lines. A fully active three-division concept with N+1 capability (i.e., the capability to meet safety requirements despite one disabled division) is incorporated in the Advanced Boiling Water Reactor (ABWR).
One alternative to the three active divisions concept is to use four active divisions. The four active divisions concept adds both more ECCS systems and supporting auxiliary systems, which require more maintenance to be performed. This is counter to the objective of reducing maintenance and improving safety.
Another alternative to the three active divisions concept is to use passive systems. Totally passive safety systems have been studied for use in BWRs because of their merits in reducing maintenance and surveillance testing of the safety-related equipment, and in eliminating the need for AC power, thereby improving the reliability of BWR operation and safety. Simplified BWRs (SBWRs) have been designed with totally passive safety features that provide more resistance to human error in accident control and mitigation.
There are, however, some tradeoffs when employing totally passive safety systems in BWRs. Due to their passive nature, the totally passive system--when designed in accordance with nuclear standards of system separation and diversity--would substantially add to plant size and cost. Therefore passive system applications to BWRs have been limited to small- and mediumsized plants having up to about 1000 MWe output.